But not all seven crossings will have lagging lefts in both directions, and some will have them only during Tempe's "peak periods," between 6:30 a.m. and 9 a.m., and 3 p.m. to 6:30 p.m., Warren said.

"This is our first time doing leading to lagging by time of day," Warren said. "Because students and faculty come from all over the Valley, we want to get the word out to as many people as possible."

Lagging lefts will be added at least in one direction and several hours a day at Rural Road at Loop 202, University Drive, Apache Boulevard, Southern Avenue, and Broadway and Baseline roads, Warren said.

The primary aim is to reduce the number of stops so that a light is not turning red as you approach it, Warren said.

Fewer stops save gas, spew less pollution and improve the quality of people's lives by getting them out from behind the wheel sooner, said Cathy Hollow, a Tempe senior civil engineer.

Another goal is to force travelers to keep to the speed limit. Warren said drivers won't be able to "barrel through" Rural Road.

The benefits of leading vs. lagging has been debated and studied for years, but Tempe engineers are studying and changing intersections on a case-by-case basis.

Adding mixed signals

The city is not the first to institute non-uniform signals to speed traffic flow, Warren said. Gilbert, Chandler and Scottsdale have also introduced the mixed signals.

"We are using the tool that best addresses each intersection," Warren said.

Commuters will now adjust from lagging to leading lefts not only from city to city but also between lights in the same town. But Warren said they will not be inconvenienced if they pay attention and wait for the light to turn green.

Those who are thrown off or confused are usually the ones who are distracted, Warren said.

Rural Road is the most congested north-south route in the city, Hollow and Warren said. It is teeming with intersections while providing access to downtown Tempe, ASU, Loop 202 and U.S. 60.

"It is one of the hardest to coordinate because of spacing of the intersections," Warren said.

Warren and Hollow make the signal switches remotely and spend hours each week traveling various routes for firsthand knowledge of how things are working.

Testing timing changes

On Tuesday, the pair traveled Rural Road between Curry and Broadway roads, driving the speed limit and investigating their efforts.

"Our goal is to (study) the progression of a vehicle going the speed limit," Hollow said.

Heading south, they rolled to a stop at Sixth Street, where the light had just turned green. The vehicles ahead hadn't switched gears yet.

"We had to slow down but at least it is green," Hollow said.

They cruised through nearly every light. Only one light was red: the signal at University Drive.

Left-turn changes

Lagging lefts will be added in at least one direction and for at least several hours a day at Rural Road. Additionally:

Loop 202 north- and southbound left turns will lag all day.

University Drive southbound left turns will lag during the morning peak, and both north- and southbound left turns will lag during the evening peak.

Apache Boulevard, both north- and southbound left turns will lag during morning and evening peak times.

Southern Avenue and Broadway Road north- and southbound left turns will lag all day.